Diaryl oxides (also referred to as diaryl ethers) are an important class of industrial materials. Diphenyl oxide (DPO), for instance, has many uses, most notably as the major component of the eutectic mixture of DPO and biphenyl, which is the standard heat transfer fluid for the concentrating solar power (CSP) industry. With the current boom in CSP has come a tightening of the supply of DPO globally and questions surrounding the sustainability of the technology have arisen.
Diaryl oxides are currently manufactured commercially via two major routes: reaction of a haloaryl compound with an aryl alcohol; or gas-phase dehydration of an aryl alcohol. The first route, for example where chlorobenzene reacts with phenol in the presence of caustic and a copper catalyst, typically leads to less pure product and requires high pressure (5000 psig), uses an expensive alloy reactor and produces stoichiometric quantities of sodium chloride.
The second route, which is a more desirable approach, accounts for the largest volume of diaryl oxides produced but requires a very active and selective catalytic material. For instance, DPO can be manufactured by the gas-phase dehydration of phenol over a thorium oxide (thoria) catalyst (e.g., U.S. Pat. No. 5,925,798). A major drawback of thoria however is its radioactive nature, which makes handling difficult and potentially costly. Furthermore, the supply of thoria globally has been largely unavailable in recent years putting at risk existing DPO manufacturers utilizing this technology. Additionally, other catalysts for the gas-phase dehydration of phenol, such as zeolite catalysts, titanium oxide, zirconium oxide and tungsten oxide, generally suffer from lower activity, significantly higher impurity content and fast catalyst deactivation.
In addition to the above routes, the synthesis of ether compounds through a mixed metal oxide catalyzed decarboxylation reaction has also been described, for instance in U.S. Pat. Nos. 5,164,497 and 5,210,322 (which references are incorporated herein by reference). These references disclose the decarboxylation of various organic carbonate compounds to form the corresponding ethers. The working examples in both references are focused on decarboxylation of non-aryl carbonate compounds. No aryl carbonate compounds are specifically illustrated in the examples and the ability of the process to decarboxylate the more hydrolytically unstable aryl carbonate compounds in favorable selectivity or yields was not recognized.
The lack of any specific examples in the above references regarding decarboxylation of diaryl carbonates is additionally not surprising since it is known from other literature that such a process is difficult to achieve. For instance, Witt et al., in Angew. Chem. Int. Ed., 1970, 9, 67, teaches that in order for the decarboxylation of diaryl carbonates to proceed, the carbonate compound must contain at least one ortho and/or para electron-withdrawing substituent. Furthermore, the reference teaches that useful yields are obtainable only from symmetrically substituted compounds (i.e., at least two electron withdrawing substituents), particularly p,p′-disubstituted, diphenyl carbonates.
Witt's teachings, that the diaryl carbonate must be substituted, are consistent with the teachings of U.S. Pat. No. 2,319,197. This reference describes a process whereby diphenylcarbonate (therefore no substitution) generates phenyl o-phenoxybenzoate (i.e. not diphenyloxide) when heated in the presence of a potassium carbonate catalyst.
With a chronic shortage of diaryl oxides such as DPO in sight, the pressing need to increase capacity, and the lack of commercially viable options, it has become crucial to develop new methods to produce diaryl oxides in a cost-effective and sustainable manner.
The problem addressed by this invention, therefore, is the provision of new processes for the manufacture of diaryl oxides.